We've just driven a development prototype of the 2012 LaCrosse fitted with the eAssist system that will be standard on the 2.4-liter four-cylinder engine, the smaller of two engines (the other is a 3.6-liter V-6).

Electric "boost"

It includes not only a start-stop function, saving gasoline when the car isn't moving, it also recaptures electric power via regenerative braking and stores it for use by an electric motor that assists the gasoline engine in short bursts.

For 2011, the 2.4-liter engine is rated by the EPA at 19 mpg city, 30 mpg highway. So an increase to 25 mpg city, 37 mpg highway would be substantial: more than 30 percent in the city, almost 25 percent on the highway.

The big question is: Does the LaCrosse with eAssist drive like a Buick?

Imperceptible start-stop

In a word, yes. The engine switching itself off and on was completely imperceptible in the prototype we drove, which engineers cautioned wasn't yet a final production vehicle.

Without looking at either of two dashboard displays or the tachometer--which has an "AutoStop" zone replacing the "0" on a standard gauge--we couldn't feel the engine switching off and didn't notice anything as it switched back on.

Acceleration wasn't swift--this is a large sedan powered by a smallish engine--but Buick claims the eAssist model is a few fractions of a second quicker to accelerate from rest to 60 mph, a statistic we couldn't verify in our half-hour drive.

Acceleration dip

The one telltale was a momentary dip in acceleration after the vehicle moved away from a stop. It was the kind of thing that might well be eliminated by the time the vehicle reaches showrooms late this year.

And Buick engineers have put a great deal of effort into making the system quiet and smooth. Engine mounts were extensively retuned, since the engine weighs more with the larger starter-alternator and the weight is distributed differently.

They even use quick pulsing of the electric motor--again, imperceptible to the driver--to compensate for engine vibrations under certain conditions. It all seems to work fine.

High-end howling

One advantage of the eAssist mild hybrid system is that it keeps engine speeds lower, by feeding in electric "boost" where the previous 2.4-liter LaCrosse engine would have had to shift down a gear to provide power.

The downside is that when full power is needed, and the transmission does shift down a gear or even two, the engine is correspondingly much louder than it otherwise has been under normal use, where it mostly sits between 1,500 and 2,000 rpm.

Revving up well past 4,000 rpm causes the engine to howl from up there under the hood. It's startling simply because the car has been so hushed and sedate otherwise.